The Soviet ballistic missile submarine K-219 suffered an accident in
the Atlantic, and later sank, in October 1986.

K-219 was of the class known in the west as Yankee I, to the
Soviets as Project 667a Navaga, arguably the first "true" Soviet
SSBN (the earlier Hotel/Project 658 class having only had the
ability to carry three missiles, something like the American
Regulus-carriers). The class was introduced in 1968, and K-219 had
been commissioned in 1971.

The principal warload of this class was sixteen R-27 Zyb missiles
(SS-N-6 Serb to NATO), each with a range of some 1,300 nautical
miles, fuelled by a hypergolic combination of unsymmetrical
dimethylhydrazine (UDMH) with nitrogen tetroxide as oxidiser (some
sources claim red fuming nitric acid instead). The alert student will
notice that each these chemicals is fairly vicious in isolation, as
well as the combination being explosive. Indeed, K-219 had already
suffered an explosion in a missile tube on a previous occasion, and
the tube had been welded shut given the impossibility of repair.

On 3 October 1986, K-219 was on a routine deterrence patrol in the
North Atlantic, some 680 miles north-east of Bermuda. At around 0514
(Moscow time, as kept on board), water was observed to be dripping
from under the plug of missile tube number six; this soon turned into
a stream. The captain ordered an ascent to safe depth, and a pump was
started to try to dry out the tube. At 0532, brown clouds of nitric
acid were observed escaping from the tube (likely to have been a
result of seawater mixing with nitrogen tetroxide). There may have
been an attempt to open the missile hatch and flood the tube; in any
case, by 0538, an explosion had occurred.

Two sailors were killed at once, and a third died from nitric acid
poisoning soon thereafter. The submarine took on water and sank
rapidly, though with all compartments sealed and sea-water pumps going
at full speed he stabilised at 300 metres. The reactor had not
automatically shut down, and enlisted seaman Sergei Preminin
volunteered to do it manually, under instruction from the chief
engineer, while wearing a full-face gas mask. He did so, but a fire in
the reactor compartment had raised the pressure there, and he was
unable to open the hatch to get back out before his air supply was
exhausted.

Captain Second Rank Igor Britanov was able to surface the submarine on
battery power, and call for help. A Soviet freighter lent assistance,
and a tow-line was attached, with the aim of bringing K-219 back to
his home port of Gadzhiyevo/Skalistiy/Murmansk-130. However, towing
attempts were unsuccessful, and vaporised nitric acid (much of it also
radioactive) continued to spread through the boat; Britanov ordered
his crew to evacuate onto the towing ship, but remained aboard
himself.

Military command in Moscow ordered the political officer, Valery
Pshenichny, to assume command, take the crew back aboard, make
repairs, and continue the patrol. Before this could be done, K-219
took on water and sank, Britanov only just escaping. There is
apparently some evidence that Britanov may have deliberately scuttled
the boat, rather than allow his crew back into a situation certain to
be deadly.

On return to the USSR, Britanov was charged with negligence, sabotage
and treason; however, after the Defence Minister Sergei Sokolov was
dismissed the next year because of the Mathias Rust incident, his
replacement Dmitriy Yazov dropped the charges.

The Soviets claimed that the damage had not been an explosion, but the
result of a collision with the Los Angeles-class submarine USS
Augusta, which was in the area; both the US Navy and Britanov denied
that a collision had taken place.

It is asserted that when the Soviets sent a hydrographic survey vessel
to examine the wreck two years later (some 18,000 feet down on the
Hatteras Abyssal Plain), several missile silo hatches had been forced
open and the missiles removed. (Glomar Explorer was built to recover
the earlier lost submarine K-129, at some 16,000 feet below the
surface. Maximum depth figures are, unsurprisingly, not available.)

Igor Kurdin, the executive officer at the time of the incident,
contributed to Hostile Waters with Peter Huchthausen and Alan White,
a narrative-style account of the incident published in 1997; the same
year, it was made into
a film of the same name, with
Rutger Hauer playing the part of Britanov, though the film changed the
story to include a collision with a fictionalised version of
Augusta. Britanov felt that the film was inaccurate and made him
look incompetent, and won the eventual lawsuit.

This is almost a case study in everything that was wrong with the Soviet military in the 1980s. Political Officers being told to take charge and continue the mission under insane conditions, sensible Captain possibly deliberately scuttling the boat to prevent this etc.

Not to mention the insanity of having liquid fuelled missiles on a submarine in the first place. Did the Soviets never develop solid rockets or could they just not afford to re-equip with them?

The radioactive gaseous nitric acid in the air is just the icing on the cake.

The R-27 was a relatively early SLBM, entering service in 1968. The Delta boats (from 1973) used the R-29 family, also UDMH/N2O4 but rather better designed (no accidents that I'm aware of, certainly no hull losses); the Typhoon boats (from 1981) used the R-39, which was solid-fuelled (mostly). I think that storability was the problem with early solid-fuelled designs.